Patent GB-809225-A (GB)
Title: Method of and installation for regulating the addition of quantities of liquid to a stream of material
IPC: G05D11/13
Filed: 1956-08-20
Abstract: 809,225. Electric correspondence control. SCHENCK MASCHINENFABRIK G.m.b.H., C. Aug. 20, 1956 [Sept. 24, 1955], No. 25440/56. Class 40 (1). A method of quantitatively controlling the addition of a liquid to a stream of material by reference to a measured quantity of said material, comprises the steps of automatically measuring the quantity of the stream of material, automatically converting such measurement into an electrical voltage which is proportional to the quantity, electrically comparing the voltage with an electrical voltage proportional to the required quantity of liquid, thereby giving rise to a differential control voltage, and using the control voltage to control automatically means for adding the liquid, so as substantially to maintain a predetermined proportion between the quantities of liquid and material. As applied to the manufacture of chip board, wood chips from a drying station (not shown) are delivered to a weighing station 2 of the type having a continuously moving band where they are continuously weighed. The weight indicating means at the weighing station controls the position of the tapping of a potentiometer 4 across which through range resistances 21, 22 a voltage is applied from a tachometer generator 5 which is coupled to the shaft of a motor 1 which drives the band of the weighing station. The chips pass from the weighing station to a mixing station 6 where liquid binding agent is added. The binding agent is delivered to the mixing station 6 by pumps 17 which are driven by a motor 13 which also drives a tachometer generator 18. A variable resistance 25 is connected in series with the generator 18 and the voltage from the generator modified by the resistance 25 in series is applied to a potentiometer 24 through an adjustable balancing resistance 23. The tapping of the potentiometer 24 and the tapping of the potentiometer 4 are connected to a null unit 20 where the voltages at the two tappings are compared and the difference voltage is used to control a motor 19 which adjusts the speed of the pumps 17 by controlling a variable ratio transmission 14 connected between the driving motor 13 and the pumps 17. The potentiometer 24 is adjusted manually to give the desired proportion by weight of binding agent to chips and if the resistance 25 was omitted the speed of the pumps 17 would be proportional to the setting of the potentiometer 24. The resistance 25 is included however since the pumps, pipes and delivery nozzles are subject to clogging and the viscosity and specific gravity of the binding agent can vary so that the weight delivered is not proportional to the speed of the pumps; the resistance 25 is automatically adjusted to compensate for these factors so that a constant weight of binding agent is delivered to the station 6. The binding agent is supplied to the pumps 17 from a container 9 which is supported by a weighing machine 10. The weight indicating means 11 of the weighing machine 10 moves the tapping of a potentiometer 12 and since a constant voltage is applied to the potentiometer from a rectifier connected to the supply mains (the exact voltage can be adjusted by resistances 43 and 44 in series with the potentiometer) the voltage at the potentiometer tapping corresponds to the weight of the container 9 and its contents at all times. The voltage at the tapping of the potentiometer 12 is applied to a null unit 45 where it is compared with the voltage at the tapping of a potentiometer 40, the potentiometer 40 being connected across the constant voltage source through adjustable resistances 41. 42. The error voltage from the null unit 45 is used to control a motor 52 which adjusts the variable resistance 25 so that the voltage across the combination of the generator 18 and variable resistance 25 in series tends to be maintained constant. This voltage is applied not only to the potentiometer 24 but also to a pulse generator 28 comprising a motor driving an apertured disc which interrupts the passage of light to a photoelectric cell and by means of which a pulse is generated for each revolution of the disc; the pulse frequency is thus proportional to the voltage applied to the motor and the pulses produced by the photoelectric cells are first amplified in an amplifier 29 and then applied to a stepping motor 30 which rotates a shaft one step for every pulse. The stepping motor drives one of the sun-wheels 36 of a differential gear 35 through gearing and the planet wheel carrying cage of the differential gear drives the tapping of the potentiometer 40; the tapping 40 is thus driven at a substantially constant speed which is proportional to the desired weight of liquid binding agent. The weight of binding agent delivered is maintained at the correct value through the control of the resistance 25 by the null unit 45 since any reduction in the weight delivered causes the tapping of the potentiometer 12 to lag behind the tapping of the potentiometer 40 and the resulting error voltage causes the tapping 25 to be adjusted so that more resistance is placed in circuit with the generator 18. The resulting reduction in the voltage across the potentiometer 24 results in the null unit 20 producing an error voltage which acts to speed up the pumps 17 and hence the voltage delivered by the generator 18; thus the pumps deliver a greater weight of binding agent and the voltage across the generator 18 and resistance 25 in series is restored to its original value. To prevent hunting of the control circuit for the resistance 25 a cyclically operating switch 51 is arranged in series with the motor 52; this switch operates to connect the null unit to the motor for one second in every minute only. When the tapping of the potentiometer 12 reaches the end of its travel during the emptying of the container 9 it actuates a switch 49 which causes an electromagnetic valve 8 of a reservoir 7 to open so that the container is re-filled with liquid binding agent; the switch 49 also controls a changeover switch 50 which disconnects the null unit 45 from the cyclical unit 51 and motor 52 and is instead connected to a motor 53 which drives the second sun-wheel 37 of the differential gear 35 so that as the tapping of the potentiometer 12 is moved to the other end of the potentiometer by the action of the weighing machine 10 during the filling of the container 9 so the tapping of the potentiometer 40 is maintained in correspondence with it; the switch 49 also gives a signal to indicate that the container 9 is being filled. When the tapping of the potentiometer 12 reaches its other end it actuates a switch 48 which closes the valve 8 and restores the switch 50 to its original position. The liquid levels in the container 9 at which filling commences and ends is determined by adjustment of the resistors 43 and 44. If the delivery nozzles from the pumps should become completely clogged the resistance 25 will be continuously readjusted so as to cause the pumps to speed up until when all the resistance has been placed in circuit a switch 27 is actuated to give an alarm. In a modification instead of moving the tapping of the potentiometer 12 by means of the weight indicating means of a weighing machine it can be moved by a pressure transducer in the bottom of the container 9. In a further modification for use in the case of liquids having a constant specific gravity the tapping of the potentiometer 12 is moved by a float. In a further modification also for use with liquids of constant specific gravity the container 9 is omitted and the pumps 17 are supplied directly from the reservoir 7 through a flowmeter 10 which controls the tapping of the potentiometer 12. In this modification also the null unit 45 can be dispensed with and in such a case the flowmeter 60 drives the opposite sun-wheel of the differential gear 35 to that driven by the stepping motor 30 and the planet wheel carrying cage of the differential gear is connected so as to vary the resistance 25 directly. The flowmeter 60 drives the sun-wheel through a pulse generator, amplifier and stepping motor.